1. Field of the Invention
The present invention relates to a semiconductor laser and a method for fabricating the semiconductor laser. More particularly, the present invention relates to a semiconductor laser capable of emitting visible light which is suitable for recording and reproducing information or for other operations for an optical recording medium, and a method for fabricating the semiconductor laser.
2. Description of the Related Art
An AlGaInP-type red-light semiconductor laser which oscillates in the 670 nanometers (nm) waveband was commercially available in 1988. The AlGaInP-type red-light semiconductor laser has hitherto been considered as an important component as a light source for an information processing apparatus such as a laser printer or an optical disk, and has been intensively studied and developed. The target of development was initially a semiconductor laser having an oscillating wavelength of the 670-680 nm range. In order to meet recent requirements such as the improvement in the visibility of a bar-code reader and the higher density of optical disk, the employed range of the oscillating wavelength is changed to be the 630 nm range which is the same level as that of the He-Ne gas laser. In future, as the storage capacity of an optical disk is increased, it will be required to realize a semiconductor laser which emits laser light from the blue and green region to the ultraviolet region which has shorter wavelengths than that of the red light. In addition, since it is possible to control the p-type conductivity of II-VI group semiconductors, the study of a II-VI group visible-light semiconductor laser rapidly progresses.
In order to shorten the oscillating wavelength, it is necessary to increase the bandgap of an active layer. However., the increase of the bandgap of the active layer results in an increase of overflow currents of electrons from the active layer to a p-type cladding layer. The overflow currents of electrons cause the temperature characteristic of a semiconductor laser to deteriorate. The deterioration of the temperature characteristic is a significant factor which limits the reduction of oscillating wavelength. In order to avoid the overflow of electrons, the composition ratio of Al in the cladding layer is increased so as to increase the bandgap between the active layer and the p-type cladding layer, or the carrier concentration of the p-type cladding layer is increased. However, in metal organic vapor phase epitaxy (MOVPE), there exists a problem in that it is difficult to dope an AlGaInP layer containing Al at a higher composition ratio with p-type impurities at a high concentration. The p-type concentration of the AlGaInP layer containing Al at a higher composition ratio is about 7.times.10.sup.17 cm.sup.-3 at the best.
Recently, as effective means for shortening the wavelength, the use of a (100) GaAs substrate which is slanted in a &lt;110&gt; direction is being intensively studied. When such a slanted substrate is used, it is possible to increase the p-type impurity concentration of the AlGaInP layer. In addition, a phenomenon, which is inherent to the AlGaInP type material, in that a natural superlattice that is inevitably formed can be suppressed. The formation of the natural superlattice may result in a reduction of the bandgap, so that the use of the slanted substrate suppresses the reduction of the bandgap.